When you use a melee or range attack/skill, your character will automatically switch to the appropriate weapon, if required. Rush Attack will switch to your melee weapon(s) if you are holding a bow, etc.
Apparently you can not use skills that require a target when paused. I just tried it, and Rush Attack and Poison Arrow do not work, even if there is a target highlighted. Selecting a skill that doesn't need a target (Healing, Whirlwind, summon creature...) will un-pause and trigger that skill, the same as using a potion. Do you have any skills mapped that don't need a target, which you can re-map to Poison Arrow?
FWIW, playing a warrior I assigned X melee attack, A range attack, Y Rush Attack, B Whirlwind, left Dragon Stone, right Dragon Morph stone (both after getting the battle tower), up Heal (level 1 comes in handy) and down summon creature (after getting it).
For potions, I generally just paused (or had the low hit point auto-pause kick in) and selected one from the menu there.
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